1. Field of the Invention
This invention is directed to a method and apparatus for processing medical imaging data using phase information, in particular to methods for location or registration of a feature of interest.
2. Description of the Prior Art
In the medical imaging field, several imaging schemes are known. For example PET (Positron Emission Tomography) is a method for imaging a subject in 3D using an injected radio-active substance which is processed in the body, typically resulting in an image indicating one or more biological functions. For example, in assessment of heart disease, a tracer can be used with scans concentrating on the perfusion of the left ventricle of the heart. PET and SPECT have been used in cardiac imaging to image perfusion and viability of the myocardium using 13NH3, 82Rb, 18FDG, 201TI, 99 mTc-Sestamibi, 99 mTc-Tetrofosm in etc.
Over the course of a dynamic scan (for example, for cardiac perfusion studies) the likelihood that a patient will move in some way is high. This motion can either be a twitch or a sudden change of position or a gradual motion as the patient relaxes over the time period of the scan. While the gross body alignment usually only changes by a small amount, this change is significant when looking at smaller organs, for example in the case of cardiac imaging. Such motion can lead to severe errors in post-processing analysis and quantification.
Consequently a motion correction (or tracking) algorithm is employed to account for these changes. Motion correction is a well understood and studied area for dynamic imaging for both medical and non-medical applications and many methods have been developed to that purpose. However, for the most part these methods assume that the appearance of the object being tracked, remains constant or stable.
However, in the case for example of imaging the Left Ventricle (LV) of the heart, for instance for the purpose of further quantitative investigation, such as Coronary Flow Reserve (CFR), this assumption is not valid: the dynamic imaging sequence involves an injection of a radionuclide agent at start of the acquisition, and the subsequent quantification relies on the analysis of the distribution of the tracer over time, into various parts of the body. As a consequence, the appearance of the LV changes dramatically over the time of the scan as the tracer diffuses through the body and the organ of interest. The tracer first enters the blood pool of the LV as it is pumped around the body; it is then washed out of the blood pool and gradually enters the myocardial muscle. FIGS. 1 and 2 show four different timepoints of the same scan indicating the large appearance changes that occur.
FIG. 1 illustrates the blood input function curve (102) of a dynamic scan indicating four timepoints: Timepoint 1 is prior to injection, Timepoint 2 is when the tracer peaks in blood pool, Timepoint 3 shows the blood having significantly washed out of the blood pool and entered the myocardium, and Timepoint 4 is the last frame when blood will be present in the myocardium, when the tracer distribution is stabilised.
FIG. 2 illustrates the left ventricle (202) at the same time points as in FIG. 1.
Conventional motion correction techniques will align the late frames well, as the appearance of the heart in these frames remains fairly constant, however they will fail on early frames where the signal changes dramatically. Therefore a method needs to be designed that will account for this change in appearance.
While this invention is not necessarily concerned with the gross changes that occur in the early frames of a typical cardiac scan, the changes that occur in the uptake in the myocardium are still significant. Therefore registration using standard intensity based techniques is not optimal.
Furthermore, as PET/SPECT imaging represents functional information, it is expected that the intensity and contrast in the image will vary significantly depending on the disease state of the patient. Moreover, various scanners, reconstruction algorithm and study protocol add more variations to image appearance and quality, and thus difficulties with registration.
These problems have been addressed in a number of ways to date:                No correction. An assumption is made that the left ventricle will not move over the time of the scan. As this assumption is usually not true, errors in the subsequent quantitative analysis will probably occur.        Segmentation-based methods: The requirement of the motion correction is to provide an accurate segmentation of the LV over the course of the scan. Therefore one method to do this is to perform a segmentation for the last frame (or a combination of the last few frames) and then update the segmentation for each new frame. This can lead to errors due to extra cardiac activity which will cause the segmentation of the LV to deform.        Registration methods: A better solution is to use a registration based scheme that estimates the motion of the LV between frames and then updates the segmentation based on that motion. This is usually performed using intensity based methods.        
Prior work has investigated phase based registration (Matthew Mellor, Michael Brady, “Phase mutual information as a similarity measure for registration”, Medical Image Analysis, Volume 9, Issue 4, August 2005, Pages 330-343) and shown its potential for use in medical imaging. However, using phase directly can be an approach limited in use.